Marzke, M W, "Evolutionary development of the human thumb,"Hand clinics 1992; 8(1): 1-8.
The bones and joints of the human thumb are a mosaic of primitive and unique features, reflecting stages in the evolution of the hand from a support element on the ground to a grasping structure in the trees and eventually to an organ dedicated entirely to manipulation. The trapeziometacarpal saddle joint configuration and associated musculature are shared with most nonhuman primate species, whereas the broad distal phalanx with its specialized palmar pad is unique to humans.
- Most of the distinctive features of the modern human thumb can be explained by the requirements for a firm grip and tolerance of large stresses associated with the use and manufacture of stone tools, which contributed for several million years to the survival of human ancestors after they returned to the ground.
- Fossil remains indicate that early members of the human family, Hominidae, had short thumbs relative to the length of the fingers, which were not subject to the large stresses associated with modern human manipulative behavior. Later hominids had very flat trapeziometacarpal joints and large distal phalanges, indicating a capacity for opposition of the thumb to all four fingertips and for tolerance of large stresses.
- Pathologies involving thumb joints contribute to the understanding of the sequence of changes in thumb morphology in the fossil record.
Doc Title: Evolution of the human hand: the role of throwing and clubbing
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The chimpanzee hand will be taken as a model for the hand of the hominid ancestor. The most ancient hominid fossils closely resemble chimpanzees, who are genetically our nearest relatives (Sibley, 1992; Ruvolo, 1997). Pan and human lineages diverged 5–7 million years ago (Mya), about the time the first hominid specimens appear in the fossil record (Klein, 1999). The fingers, metacarpal and carpal bones of the chimpanzee hand are elongated, but in typical primate fashion the thumb is small, weak and relatively immobile (Figs 1 and 2). The third and fourth metacarpals, which absorb the highest compression during knuckle-walking, are especially robust (Lewis, 1977; Susman, 1979). Both proximal and middle phalanges are curved toward the palm to withstand stress from gripping limbs during arboreal locomotion (Susman & Creel, 1979; Susman, 1987, 1994). The finger tips are cone-shaped, and lack broad apical tufts (Napier, 1960; Susman, 1988b, 1991). Owing to the transverse arrangement of the metacarpo-phalangeal articulations, there is a transverse skin crease across the palm (Napier, 1960, 1993; Lewis, 1977; Fig. 1). Thumb phalanges and metacarpals are slender and short (Susman, 1994; Fig. 2) and the intrinsic muscles of the thumb, underlying the thenar region of the palm, are small (Marzke et al. 1992).
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The palm has several derived features. Because the fourth and fifth metacarpals are progressively shorter than the third, there is an obliquity to the hand when it is flexed. This produces flexure creases that run obliquely, from the lower ulnar side of the palm to the upper radial side (Napier, 1993). The thenar and hypothenar eminences are enlarged by fat pads which overlie the muscles. Contraction of the palmaris brevis muscle stiffens the hypothenar pad (Marzke et al. 1992). Several features increase the ability of the centre of the palm to withstand stress imposed along the second and third fingers (Marzke & Marzke, 1987). The metacarpals and bases of the proximal phalanges of these fingers are robust. A palmar fat pad in the third metacarpal region protects the deep branch of the ulnar nerve. Stability of the third metacarpal base is enhanced by a styloid process on its dorsal radial aspect. When the finger is extended, the styloid process locks the carpal and metacarpal bones together, preventing hyperextension. A ligament from the pisiform bone to the third metacarpal base further restrains hyperextension (Marzke & Marzke, 1987).
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For efficient throwing the hand must be able to grip the missile while energy is transmitted to it, then accurately control its release. This requires a fingertip grip. The thumb must be long enough and sufficiently mobile to oppose its fingertip pad to the missile on one side while the fingers oppose their distal pads to the opposite side and adjust themselves to irregularities in naturally occurring rock spheroids. For accurate release, the fingers must be under precise neural control and able to absorb without injury the reaction force resulting from the propulsive thrust.
These adaptations are all found in the human hand. The thumb has lengthened and can be fully opposed to the fingers, which have shortened. The thumb and the first two fingers, which play the major role in the throwing grip, are strong and robust. Thumb opposition is enhanced by addition of a muscle that flexes the terminal phalanx, and is matched by rotation of the fingers as they flex: supination on the ulnar side, pronation on the radial side – exactly as needed for a fingertip grip of a sphere. Broad apical phalangeal tufts support soft, fleshy fingertip pads that adapt themselves to irregular spheroids and provide a large friction surface. The fingertips are highly innervated with sensory endings that inform the brain about the missile and forces acting on it. Precise neuromotor control of finger muscles permits submillisecond release times needed for throwing accuracy. When a missile is released, there is only one point on the arc of the moving hand where release will result in movement towards the target (Hore et al. 1995). Abduction of the thumb and extension of the finger joints control this action. A 1-ms delay in finger extension causes a change in direction of 2.2° (Hore et al. 1996a,b). A baseball pitcher must regulate ball release with a tolerance of less than 0.5 ms to deliver the missile within the strike zone. Enhanced control of the hand, a key element in throwing accuracy, is reflected in expanded representation of the fingers in the human sensory and motor cortex (Napier, 1965).
Kinetic energy transferred to the missile is channelled through the index and middle fingers of the throwing hand. At release, the thumb drops away, these fingers extend and their apical tips provide the final thrust. An equal and opposite reaction force acts to hyperextend these fingers, particularly the third finger, which due to its length is the last to lose contact with the missile (House, 1994; Hore et al. 1996b). Stress travels down the phalanges through the metacarpals in the palm to the carpal bones where it is dissipated. The robustness of the second and third fingers which absorb stress, the styloid process and ligamentous stabilization of the third metacarpal which prevent hyperextension, and the deep palmar fat pad which shields the ulnar nerve all contribute to protecting the hand against throwing injury.
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ther adaptations are specific to the clubbing grip. One of these is the slant of the metacarpal–phalangeal articulations. When the fingers are partially flexed, they form an oblique line. Together with the partially flexed thumb, a corridor is formed – a cylindrical cavity lying diagonally across the palm. When a club is squeezed tightly against the palm, this anatomical configuration assures that it is positioned in an oblique manner. On the ulnar side, the implement is clamped against the hypothenar fat pad, stiffened by contraction of the palmaris brevis muscle, while the thenar musculature and its subcutaneous fat layer buttress the radial side. When a club is swung, the wrist deviates in the ulnar direction just before impact. Combined with the oblique angle of the grip, this aligns the club with the forearm, increasing the radius of arm-plus-club and the velocity of the club, thereby providing maximal mechanical advantage (Marzke et al. 1992).
At impact, the reaction force acts to drive the club in a direction opposite to its former trajectory. The base of the handle exerts pressure on the base of the fingers on the ulnar side of the hand, whereas the apical end of the handle is driven against the radial side. If the grip is to be maintained, these two parts of the hand must be capable of withstanding the stress of impact. On the ulnar side, the base of the fifth finger absorbs much of the impact. Its metacarpal has thickened and its base has enlarged. The thumb stabilizes the clubhandle on the radial side. Modified carpal bones on the radial side help dissipate stresses generated in the thumb during clubbing (Lewis, 1977, 1989; Marzke et al. 1992). The thumb is critical for ‘hanging on tight’ (House, 1994; Ohman et al. 1995; Welch et al. 1995). Its robustness and muscularity are adaptations for power clubbing.
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Among hand authorities, the general view is that the human hand is adapted for tool behaviour (Susman, 1994). Special emphasis has been given to the fabrication and use of stone implements (Kortlandt, 1986; Marzke & Shackley, 1986; Marzke, 1992b; Napier, 1993; Marzke & Marzke, 2000). Stress from pounding with hammerstones might account for many features of the hand (Marzke & Marzke, 1987; Marzke, 1992a; Marzke & Wullstein, 1996). However, the throwing grip is not recruited during stone tool-making (Marzke et al. 1998), submillisecond control of the release of a hand-held rock is irrelevant for such purposes and the clubbing grip is useless for flaking stone in the manner used by early hominids (Marzke & Shackley, 1986; Marzke, 1992a, 1997). Furthermore, the hand of A. afarensis (described below), which is dated at 3.2 Mya, shows many features of the modern human hand, yet antedates the earliest identified stone tools (2.6 Mya). When such artefacts first appear, the hominid hand had already closely approached its current state (Susman, 1988a,b, 1991, 1993). Adaption for improved throwing and clubbing would have pre-adapted the hand for stone tool knapping.
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Doc Title: Evolution of human opposable thumb
Scientists have discovered a gene enhancer, known as HACNS1, that may have contributed to the evolution of the uniquely opposable human thumb, and possibly also modifications in the ankle or foot that allow humans to walk on two legs, according to a paper published in Science on Sept. 5, 2008.
This study is the first to provide evidence of the existence of human-specific gene enhancers, which are switches near genes in the human genome.
Dr. Shyam Prabhakar, first author of the paper and Senior Research Scientist at the Genome Institute of Singapore (GIS), said, "Opposable thumbs, manual dexterity and ankle or foot adaptations for walking on two legs are hallmarks of our species. We think we may have discovered one of the pieces of the genome that encodes some of these definitive human traits.
"This is just the first step - we need to characterise HACNS1 in more detail, and also test the hundreds of other HACNSs we have identified in the genome to figure out what role, if any, they play in making us human," he added.
The opposability of the human thumb is its unique ability to swing toward the palm and oppose the other four fingers to provide a tighter and more precise grip on objects.
The surprising complexity and abundance of enhancers, which turn on genes in the appropriate cells, have only recently been appreciated. Evolutionary changes in the DNA sequence of enhancers are thought to have triggered changes in human development that make us different from chimpanzees and other apes. Thus, the many observable differences between humans and chimpanzees, such as brain size, hair density, tooth patterns, pelvic structure and hand and foot modifications, could have arisen partly through changes in the way developmental genes are turned on.
The discovery provides significant insights into the genetic differences between humans and chimpanzees - the species that is approximately 99 percent similar to humans in terms of genetic composition. Apart from the obvious evolutionary interest, a more practical goal of such research is a more complete molecular understanding of the human body, leading eventually to a better understanding of human diseases and their treatments.
On a hunt for enhancers that could make us human, the authors of this study zoomed in on a genomic region they termed human-accelerated conserved non-coding sequence 1 (HACNS1).
HACNS1 showed statistical signatures of being an enhancer, and also had the most surprising amount of sequence change during human evolution of all the 110,000 such sequences identified in the human genome - it was by far the most striking candidate.
Remarkably, HACNS1 was found to play a unique human-specific gene-activating role in a region of the developing limb that eventually forms the junction of the wrist and thumb, and also extends partially into the developing thumb. A similar, though weaker activating role was also observed in the corresponding ankle/foot-forming regions of the developing hind limbs.
Highlighting the practical long-term goal of their joint project, Dr. James P. Noonan, last author and Assistant Professor at Yale University, pointed out, "Insights into human diseases and their treatments are often obtained through studies in non-human 'model organisms' such as mice. However, many human diseases are not reproducible in mice, and some diseases such as Alzheimer's and HIV/AIDS are not even known to exist in chimpanzees, our closest 'relatives'. Moreover, even if a disease is observable in a model organism, inter-species differences often cause treatments that appear to work when tested on, say, lab mice, to fail at the stage of human clinical trials. It is therefore imperative for human medicine that we fill in the gaps between our species and others by comprehensively characterising human-specific genomic sequences and molecular processes. For this reason, it is important that we understand, at a molecular level, what it means to be human."
Principal Investigator at Singapore's Institute of Molecular and Cell Biology (IMCB), Associate Professor Venkatesh Byrappa added, "This is an elegant demonstration that changes in the gene regulatory region have actually led to a novel function unique to humans. These changes might be associated with morphological innovations that distinguish humans from other primates."
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